frankly, any mission where we send anybody to another planet in our solar system is going to end up being a one-way trip, even Mars. the moon was barely doable with 1960s/1970s technology. we would need to devise a spaceship that can house multiple human beings for years; those human beings would need to be able to live almost literally in each other's laps with zero ability to get away from each other, except for EVAs, the entire time. and if anything goes wrong, there is literally nothing we would be able to do about it unless it happened while the ship was near Earth. in short, any mission is going to have to be 100% self-sufficient and capable of solving problems on its own.
in order for that to change, we would need new propulsion systems capable of significant thrust in a short amount of time, in space, with minimal fuel requirements that can be manufactured from resources on Earth. ideally we'd be able to capture an asteroid or comet and process it so that fuel can be generated in space, which would help a lot, but the technology for all of that also doesn't exist yet.
and even assuming we can solve all of those problems, we'd still need to solve the problem of maintaining life in space without supplies from Earth before any sustained habitation could happen.
and that's all challenges we have to overcome if we want to expand within the solar system.
futurebird